The present invention relates to a laser scanning system, more specifically, a beam position detecting structure for detecting a beam position in the scan line of a laser beam.
Recently, the laser printer has become popular as an image forming apparatus, and well known is its function of forming a hard copy of image data onto a recording sheet with use of a so-called electrophotographic image forming process.
The electrophotographic image forming process may include the following steps:
the circumferential surface of a photoconductive drum is uniformly charged with a corona charger; PA1 the surface of the photoconductive drum is then scanned with the laser beam which is ON/OFF modulated in accordance with the image data to be printed; PA1 electrostatic voltage at the exposed portion of the photoconductive drum is decreased and a latent image is formed; PA1 charged toner is attracted to the discharged position and the latent image is developed; PA1 the developed latent image, or toner image is electrically attracted and transferred to a recording sheet; and PA1 the transferred toner image is then fused and fixed by being heated/pressed at a fixing unit. PA1 a laser source for emitting a laser beam; PA1 a deflecting device for deflecting the laser beam emitted by the laser source so that the laser beam repeatedly scans at least a predetermined angle zone; PA1 a light receiving device for receiving the deflected laser beam before the predetermined angle zone 1s scanned by the deflected laser beam; and PA1 a mounting member for mounting the laser source and the light receiving device. PA1 a laser source for emitting a laser beam; PA1 a deflecting device for deflecting the laser beam emitted by the laser source so that the laser beam repeatedly scans at least a predetermined angle zone; and PA1 a light receiving device for receiving the deflected laser beam before the predetermined angle zone is scanned by the deflected laser beam, wherein the laser source and the light receiving device are arranged on the same side with respect to the predetermined angle zone.
Also becoming popular is a laser photo-plotter, which depicts an original plate, such as a printed board, on a surface of a photoconductive material by using a laser beam scanning method.
In a laser beam scanning system, a laser beam emitted from a semiconductor laser is deflected by a deflecting device and then scans a predetermined angle zone.
In the prior art, there have been provided various methods for deflecting the laser beam and performing the scanning of the laser beam. Among them, generally known is a method wherein the laser beam scanning is performed by rotating a rotatable mirror, such as a polygon mirror, which has a plurality of reflection surfaces, or a galvanometer. For such methods, utilization of a semiconductor laser as a laser source is becoming popular and well recognized, since the semiconductor laser make it possible to make the apparatus compact in size and to reduce production cost.
FIG. 9 shows an example of a known laser scanning system. In the laser scanning system, laser beam LB is emitted from a semiconductor laser 110 and deflected by a reflection surface 101A formed on a side wall of a polygon mirror 101. The polygon mirror 101 is rotatably driven to change the angle of the reflection surface so that the laser beam LB is deflected to repeatedly scan a predetermined angle zone X which corresponds to the image-forming zone of the circumferential surface of a photoconductive drum 103.
In laser beam printers, it is necessary to synchronize a modulation of the laser beam with its scanning operation. Therefore, in the laser scanning system of FIG. 9, there is provided a photo sensor 106 for detecting the laser beam LB in order to adjust the starting point of ON/OFF modulation of the laser beam in each scanning (in each scan line). The photo sensor 106 is arranged at the edge portion outside of the predetermined angle zone X so as not to affect the formation of a latent image on the circumferential surface of the photoconductive drum 103.
Further, a reflection mirror 105 is provided for directing the deflected laser beam LB to the photo sensor 106 before the laser beam LB scans the predetermined angle zone X. In order not to affect the image formed on the circumferential surface of the photoconductive drum 103, the reflection mirror 105 and the photo sensor 106 are oppositely disposed with respect to the predetermined angle zone X. In other words, the reflection mirror 105 and the photo sensor 106 are arranged on the leading end and trailing end sides outside of the predetermined angle zone X, when seen in the scanning direction of the laser beam LB, respectively. In this case, the reflection mirror 105 is arranged such that the optical path lengths between the semiconductor laser 110 and the circumferential surface of the photoconductive drum 103 so that it, equals that between the semiconductor laser 110 and the photo sensor 106.
The photo sensor 106 receives the laser beam LB, reflected by the reflection mirror 105, and generates a signal indicating a detection of the laser beam LB. Thus, the signal generated by the photo sensor 106 is utilized as a horizontal synchronous signal to be transmitted to a horizontal synchronization detecting circuit. Namely, the beam position of the scanning laser beam LB can be determine upon the detection of the laser beam LB by the photo sensor 106 by way of the reflection mirror 105. The latent image is formed from a predetermined position on the circumferential surface of the photoconductive drum 103 in accordance with the signal obtained by the photo sensor.
In the laser scanning system described as above, the semiconductor laser 110 and the photo sensor 106 are installed on respective base plates 113 and 161, which are fixedly disposed in a housing (not shown). The base plate 113, installing the semiconductor 110, is further equipped with a driving circuit 121 for driving the semiconductor laser 110, and an automatic power control circuit 122 for feedback-controlling the output of the semiconductor laser 110. While the other base plate 161, installing the photo sensor 106, is further equipped with a synchronous signal detecting circuit 123.
Reference numeral 102 denotes an f.theta. lens as a scanning lens which corrects the scanning speed of the laser beam LB so that the scanning speed on the circumferential surface of the photoconductive drum 103 is proportional to the deflected angle of the laser beam LB. Reference numeral 104 is a cylindrical lens which focuses the laser beam LB on a line perpendicular to a rotational axis of the polygon mirror 101 in order to minimize the affect of the tilt of the reflection surface 101A of the polygon mirror 101.
However, in such an arrangement for detecting a laser beam scanning position, it should be noted that both a mirror for reflecting the laser beam and a photo sensor for detecting the laser beam are independently provided in the system. Accordingly, the number of parts increases and the apparatus itself becomes structurally complicated. Furthermore, each of the increased number of parts has to be accurately installed in the apparatus. Therefore, such a structure would cause a problem from the viewpoint of increased total parts cost and installation cost.
There is also known another laser scanning system, In which the photo sensor and the control unit are provided on the same base plate. An optical fiber is provided such that one end of the optical fiber is placed at the optically equivalent position to the circumferential surface of the photoconductive drum, and the laser beam, emitted at the other end of the optical fiber, is received by the photo sensor. Thus, the laser beam reflected by the reflection device is received by the photo sensor by way of the optical fiber.
In accordance with this arrangement, when compared with the laser scanning system of FIG. 9, it becomes possible to install the synchronous signal detecting circuit and the control unit on the same base plate. Therefore,the base plate provided for mounting the photo sensor In the scanning system of FIG. 9 can be omitted. That is, the base plate in this second example can serve as not only the base plate for the control unit but the base plate for the photo sensor. It results in that the installation of the overall system is simpler and the overall production cost can be reduced.
However, even in this arrangement, there is a problem in that a part of the laser beam leaks out of the optical fiber, and the total quantity of light transmitted to the photo sensor becomes insufficient to accurately detect the laser beam.